The present invention relates to a method of obtaining a developed two-dimensional image of the wall of a borehole.
It is already known to obtain two-dimensional images of certain physical magnitudes referred to below as xe2x80x9cprimaryxe2x80x9d magnitudes as developed over the wall of a borehole on the basis of measurements of such a magnitude that are obtained at each depth from within the borehole and for each azimuth direction. It is thus possible to reconstruct a developed image of said physical magnitude by causing each pixel of a plane defined by depth and azimuth coordinates to have a color (or a gray level) that is a function of the value of the physical magnitude measured at that point.
By way of example, two-dimensional images have already been made of the electrical conductivity of the ground surrounding the borehole, and more particularly of its resistivity in the immediate proximity of the borehole wall (FMI imaging). Other physical magnitudes can be envisaged, insofar as it is possible to measure them at a given depth and over a plurality of azimuths, as is the case for example of acoustic impedance measured by means of an ultrasound imaging device (UBI).
Nevertheless, until now it has not been possible to obtain such two-dimensional images for certain physical magnitudes when it is not possible at a given depth to measure that magnitude in only a single azimuth direction (possibly integrated over an azimuth window of varying size depending on the type of measurement). For that kind of physical magnitude, all that is obtained is a curve, known as a xe2x80x9clogxe2x80x9d, showing how the magnitude varies as a function of depth in an azimuth direction that may be fixed or otherwise.
As an example of such a xe2x80x9csecondaryxe2x80x9d physical magnitude, mention can be made of density or of photoelectric factor as measured by means of a high resolution measurement device. The same applies to permeability obtained from a nuclear magnetic resonance measurement device, or to dielectric constant or to wave attenuation recorded by means of an electromagnetic propagation measuring tool. All such measurement devices are mounted on a tool having pads pressed against the wall of the borehole in a given direction and thus providing, at each depth, a value of the physical magnitude in question as measured in said direction.
These measurements therefore suffer inherently from a lack of coverage as a function of azimuth, and that constitutes a limitation in formations that are not uniform, e.g. of the nodular, lenticular, conglomerated, or fractured types.
The present invention seeks to provide a method enabling a two-dimensional image of such a physical magnitude to be reconstructed from linear sampling of said magnitude as a function of depth in a borehole, i.e. from a log.
To this end, in a first aspect, the invention provides a method of obtaining a developed two-dimensiotnal image of the wall of a borehole, the method being characterized by the fact that it comprises the steps consisting in:
establishing a relationship between a primary physical magnitude measured in said borehole as a function both of depth and of azimuth, and a secondary physical magnitude measured as a function of depth; and
deducing from said relationship, values for said secondary physical magnitude as a function both of depth and of azimuth.
Thus, by using the values of the secondary physical magnitude as deduced as a function of azimuth, it is possible to reconstruct an image of said magnitude which, until now, has been known from a curve established as a function of depth only,
In a particular implementation, the method includes a stop consisting in matching the resolution of the primary physical magnitude to the resolution of the secondary physical magnitude.
Also in a particular implementation, the method of the invention includes a step consisting in matching the angle reference for the measurements of the primary physical magnitude with the angle reference for the measurements of the secondary physical magnitude.
Also in a particular implementation, the method of the invention includes a step consisting in matching the depth values at which the primary physical magnitude is sampled with the depth values at which the secondary physical magnitude is sampled.
The various above-mentioned matching steps seek essentially to correct certain measurement artifacts caused by variations in speed due to imperfect control of the movement of the sondes carrying the measurement instruments as they move in the borehole. This applies in particular to depth matching which is made necessary by the elasticity of the cables that support the sondes and by the friction between the sondes and the walls of the borehole. This also applies to matching angular references due to the fact that sonde azimuth varies during vertical displacement.
The purpose of matching measurement resolution is to ensure that measurements performed on different physical magnitudes and thus known with different resolutions are made comparable. For example, it is clear that resistivity is measured with depth resolution that is much higher than that of density.
In a particular implementation, said relationship is established between the values for the secondary physical magnitude and the values for the primary physical magnitude as measured on the azimuth direction in which the secondary physical magnitude is measured.
Advantageously, said relationship is established subject to values for at least one auxiliary physical magnitude.
In which case, said auxiliary physical magnitude is sampled as a function of depth and is integrated over at least one azimuth interval.
These auxiliary physical magnitudes as measured or as calculated at each depth step provide the context for local interpretation of the relationship between the primary and secondary physical magnitudes. It is assumed that they are appropriately matched in depth to both of said physical magnitudes.
As an example of such auxiliary physical magnitudes, account can be taken of indicators of porosity such as the results of gamma ray measurement or of neutron logging, or account can be taken of texture indicators such as statistics derived from analyzing borehole images.
More particularly, said relationship is established by means of an artificial neural network.
Naturally, that is but one particular computation technique which has been found to be advantageous in the cases under consideration, and other methods can be envisaged.
In a particular implementation, in order to deduce the values of said secondary physical magnitude from said relationship, said relationship is applied to the values of said primary physical magnitude sampled as a function of depth and of azimuth.
Also in a particular implementation, the measured values of the secondary physical magnitude are compared with the corresponding reconstructed values, and a criterion concerning the quality of the reconstruction model is deduced therefrom, in particular concerning the quality of the neural model.
This comparison can be performed by any appropriate method, e.g. by the least squares method. It is then possible to readjust the model to optimize the quality criterion.
In another aspect, the invention provides a method of obtaining a developed two-dimensional image of the wall of a borehole, characterized by the fact that it comprises the steps consisting in:
measuring a primary physical magnitude in said borehole as a function both of depth and of azimuth;
measuring a secondary physical magnitude in said borehole as a function of depth;
establishing a relationship between said primary physical magnitude and said secondary physical magnitude; and
deducing from said relationship values for said secondary physical magnitude as a function both or depth and of azimuth.